Feature / Application Context

[chaoos]

This PR is a preparation for developments required for an implementation of the PTBC algorithm. It replaces in the whole codebase the MPI communicator MPI_COMM_WORLD with a variable app()->mpi.comm.

It adds an application context struct that may hold information about the current application instance, such as the MPI communicator (as of now). I might be extended in the future.

There is an MPI header wrapper, such that every translation unit that does

#include <mpi.h>

will include the new header file instead. This header includes the true mpi.h header and declares some functionality:

• app(): returns the current application context, by default mpi.comm is MPI_COMM_WORLD.
• app()->ptbc.initialize(): (optional) initializes the application context with a certain MPI communicator.

In future developments, this may be extended with a world_comm member that is a duplicate of MPI_COMM_WORLD. With this communicator global communication can take place, while the rest of the application uses app()->mpi.comm.

This PR is based on top of cmake_support to benefit from the cmake build system. Its PR #664 has to be merged first and this branch has to be regularly rebased on cmake_support. How to:

$ git remote -v
fork    https://github.com/mtaillefumier/tmLQCD (fetch)
fork    https://github.com/mtaillefumier/tmLQCD (push)
origin  https://github.com/etmc/tmLQCD.git (fetch)
origin  https://github.com/etmc/tmLQCD.git (push)

Then:

git fetch fork
git checkout feature/app_context
git rebase fork/cmake_support

TODO:

• pass QUDA the proper communicator
• pass DDalphaAMG the proper communicator
• pass QPHIx the proper communicator

Alternatively one could of course replace #include <mpi.h> with #include "mpi.h" to make the wrapping explicit. That would maybe be better ...